1. Field of the Invention
The present invention relates to an antenna module that is mounted on the mother board of an electronic apparatus such as a personal computer. In particular, the invention relates to an antenna module that is suitable for short-distance radio data communication.
2. Description of the Related Art
In recent years, radio data transfer methods that enable short-distance information exchange by transmitting and receiving radio waves in a prescribed frequency band (e.g., a 2.4-GHZ band) have come to attract much attention. With the spread of such a technology, in the future, it will become more necessary to provide an antenna module for transmission and reception of communication data on the mother board of an electronic apparatus such as a personal computer.
FIG. 2 illustrates a conventional antenna module of the above kind. As shown in FIG. 2, the antenna module is generally configured in such a manner that a chip-type antenna 2, a circuit unit 3, and a connector 4 are mounted on an insulative substrate 1 that is mounted on the mother board (not shown) of a personal computer or the like and that a ground conductor 5 is formed on the back surface of the insulative substrate 1. In the chip-type antenna 2, a band-shaped conductor 2b is formed spirally on the surface of a chip-shaped dielectric member 2a. The spiral, band-shaped conductor 2b is connected to a feeder line 6. One end, near the feeding point, of the spiral, band-shaped conductor 2b is connected to the ground conductor 5 through a through-hole (not shown). The overall length of the spiral, band-shaped conductor 2b is set slightly shorter than ¼ of the free space wavelength λ of radio waves used, that is, λ/4, with wavelength shortening by the dielectric member 2a taken into consideration. The circuit unit 3 is such that a transmission/reception circuit in which electronic parts such as amplifiers and an oscillator are arranged is covered with a shield case. The transmission/reception circuit is connected to the feeder line 6. The connector 4 is to connect, to a mother-board-side external circuit, lead lines leading from the transmission/reception circuit of the circuit unit 3.
The above conventional antenna module functions as what is called a monopole antenna in which one end of the spiral, band-shaped conductor 2b is grounded. That is, it utilizes a mirror principle: the spiral, band-shaped conductor 2b resonates as if an equivalent radiation element existed on the opposite side of the ground conductor 5. Therefore, unlike the case of a dipole antenna, it is not necessary to provide a pair of radiation elements. The antenna module is fabricated in this manner to decrease the occupation area of the radiation element on the insulative substrate 1 and hence the entire module can easily be miniaturized.
Compactness has become increasingly important for antenna modules to permit mounting of an antenna module on a mother board of an electronic apparatus such as a personal computer. The compactness of the above conventional antenna module is satisfactory. However, the very reason that the above conventional antenna module is compact enough to be useful, i.e. the antenna module is of a monopole type that utilizes a mirror principle in which the presence of the ground conductor 5 allows the spiral, band-shaped conductor 2b to resonate in the same manner as in a half-wave dipole, creates a problem. More specifically, because the resonance frequency of the antenna module shown in FIG. 2 depends on the positional relationship between the chip-type antenna 2 and the ground conductor 5, the antenna characteristics are affected by mother-board-side ground conductor. Moreover, as the ground conductor is relatively close to the antenna module, the antenna characteristics are highly sensitive to the position of the ground conductor with respect to the antenna module. This means that it is difficult to attain high reliability in both manufacturability, in addition to operability as it is not unlikely that the relative position may vary significantly, for example due to temperature fluctuations.